Sound absorbing panel for interior walls
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to sound absorbing panels and, in particular, to perforated sound absorbing panels. 
     Description of the Prior Art 
     Perforated sound absorbing panels are well known in state of the art paneling. Such panels function on the basis of the Helmholtz resonator principle. The sound absorbing effect of the perforated panels is based on the circumstance that a significant portion of incident acoustic energy is absorbed but not reflected. The panels allow sound to pass through holes into the interior of the wall, but prevent a part of the sound that has penetrated from emerging again. 
     A perforated panel in the form of a veneered particle board is also known. The particle board has blind holes of relatively large radius, while the veneer has relatively small holes which direct the incident sound into the blind holes. This veneered particle board presents the difficulty first in that it is costly and labor-intensive, and secondly that the holes are easily fouled and for all practical purposes cannot be cleaned. The fact that the panel is almost inflexible limits its application. 
     SUMMARY OF THE INVENTION 
     An object of the invention is the creation of a sound absorbing panel that not only is characterized by excellent sound absorption properties but allows a large number of variations in architectural styling. 
     The panel also exhibits the Helmholtz effect, but avoids costly hole drilling. Measurements in a Kundt tube have yielded an average absorption factor α of 0.55. In the 500 to 2,000 Hz range the absorption rises from 0.6 to 0.9. Surprising is that micro fine holes, approximately 1.2 mm diameter, open surface approximately 1.5 percent, having considerably better absorbing capacity than drilled holes approximately 6.5 mm diameter, open surface about 20 percent. The panel as described and claimed in the present invention thus exhibits excellent acoustic properties at the high frequencies which for the most part are found to be especially disturbing. The milled faces allow shaping and bending of the panel without closing of the holes. Consequently, curved wall surfaces can also be faced with shaped or bent panels. The relatively small holes are barely visible, this being an effect often desired. 
     Different holes sizes can be obtained by simple means in a single panel by means of different groove depths. The open face amounts preferably to about 2 percent, while the diameter of the holes is, for example, approximately 1 mm. A panel such as this reflects incident light well and is also suitable as a light reflector, for example, above a workplace containing noise generating machines, such as a computer room. 
     In one embodiment of the invention, the grooves are funnel-shaped in cross-section. This panel is largely self-cleaning. Production of such holes is especially simple when the holes are V-shaped in cross-section. 
     In a preferred embodiment, the grooves are V-shaped in the bottom area of the groove, and the grooves have more or less parallel sides. The spacing between the grooves can be made smaller than in the case of V-shaped grooves, and accordingly, more holes can be produced per unit area. 
     Additional advantageous characteristics are illustrated by the following description and the drawings. Two examples of embodiments of the invention are explained as follows with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b show partial views of the narrow side of a panel in first and second embodiments of the invention. 
     FIG. 2 shows a view of the panel in FIG. 1a as seen in the direction of arrow II. 
     FIG. 3 shows a view of the panel in FIG. 1a as seen in the direction of arrow III. 
     FIG. 4 presents a partial sectional view through the panel in FIG. 1b. 
     FIG. 5 is a diagrammatic representation of a partly bent panel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The panel shown in FIG. 1a is a plywood panel approximately 7 mm thick. However, the panel may also consist of a suitable plastic. Greater or smaller thicknesses are, of course, also possible. As is shown by FIG. 1a in conjunction with FIGS. 2 and 3, several parallel and straight grooves 3a and 3b are incorporated in surfaces 20a and 20b. The groove depth is about 4 mm, so that square openings 5 are present at the intersections of grooves of different surfaces. As is seen from FIG. 1a, the grooves have parallel side surfaces 22 and intersecting surfaces 21. The surfaces 21 form the groove bottom and intersect at an angle of approximately 90°. 
     FIG. 1b shows another plate, one in which the grooves 4a and 4b are V-shaped in cross-section. The side surfaces intersect at an angle substantially smaller than 90°. In addition to the grooves shown here, other groove shapes are conceivable, for example, grooves U-shaped in cross-section. The grooves may also be curved or arranged in a zigzag pattern. 
     FIG. 4 shows a panel that is fastened, for example, on a ceiling 12 as ceiling liner. Sound striking the panel in the direction of the arrow 7 passes through the holes 6 into the grooves 4a. The sound, represented here diagrammatically by the lines 8, is absorbed by the material surrounding the grooves 4a. It goes without saying that additional panels as described in the present invention or other sound absorbing panels can be mounted between the panel 2 and the ceiling 12. The panel 1 or 2 is preferably backed with glass wool, rock wool, or the like. The sound emerging behind panel 1 or 2 is dissipated by the funnel-shaped groove, and as a result can penetrate the sound absorbing layer mounted behind the panel to particularly great effect. 
     It has been found that low-pitched sounds can be absorbed by resonance in the panel disclosed in the present invention. 
     As is shown in purely diagrammatical form in FIG. 5, the panel 2 may be partially or completely bent. The panels 1 and 2 described in the present invention are very flexible, like a panel of rubber, because of the intersecting grooves. One essential feature of the invention is that the panels 1 and 2 can be the same in design on both sides, thus making manufacture and use considerably easier. The grooves in the front surface may also be shallower than the ones in the rear surface, or may even be replaced by through holes. 
     In addition to the considerable advantages of the panel discussed above, it is to be seen that the panel is very simple and its production is marked by low material intensity enabling a sound absorbing panel to be constructed, which reflects light well, that both makes excellent allowance for acoustic requirements and, because of its flexibility, is versatile in use in architectural styling.